1. Field of the Invention
The present invention relates to low, normal, and high velocity absorbers generally and more particularly to air inlet designs for same.
2. Description of the Related Art
Power generation units require desulfurization and use high velocity absorbers for same because they offer economic advantages such as less capital costs, less real estate requirements, allow shorter more compact absorbers, and provide improved SO.sub.2 removal efficiency. On the other hand, high velocity has some disadvantages such as increased resistance to the gas flow, increased sensitivity of the system to changes in the hydraulic behavior of the gas and liquid phases, and especially sensitivity to low L/G ratio.
Physical model studies show that the gas velocity through the inlet and the absorber L/G greatly affects the gas distribution and mist eliminator performance of the absorber. Conventional absorber inlet designs providing gas velocities at or below 3000 feet per minute, result in good gas distribution across the absorber at moderate L/G ratio and with the gas flow in the absorber held in the range of 1 to 12.5 feet per second. At these operating parameters the liquid curtain falling off the protective awnings above the inlet provide protection against inlet wetness at low load operation and provide ample resistance to distribute the gas across the absorber cross section. Any gas flow distortion can be tolerated at these conditions because of the large margin in gas velocity before mist eliminator failure.
As gas velocity in the absorber increases above 12.5 feet per second and approach 20 feet per second, the influence of the L/G diminishes, the resistance of the liquid curtain falling off the awning becomes significant and magnifies the effects of gas flow distortion. A droplet as large as 1/4 inch in diameter can be carried up in the absorber gas stream at a gas velocity of 20 feet per second.
While the liquid curtain is needed to humidify and help gas redistribution it causes two significant problems:
1. Significantly increases the inlet pressure drop. PA1 2. Distorts the flow pattern as the gas rises through the absorber.
In the new generation of high velocity absorbers, gas velocity is set between 15 and 20 feet per second. Minor distortion in the gas flow pattern results in localized gas velocities approaching the failure limits of the mist eliminator as the gas velocity approaches or exceeds the 20 feet per second limit.
To overcome the negative effects of the high inlet gas velocity, prior art absorbers used increased inlet flow areas and limited the gas velocity to the conventional 3000 feet per minute in the absorber. This solution while simple and practical resulted in a larger inlet aspect ratio and increased the absorber height. Other prior art designs used advanced low pressure drop gas inlets or used the available means within the system to redistribute the gas flow without significant increase in the inlet's resistance.
As may be best seen in FIGS. 1 and 2, prior art absorber inlets may vary in shape and size but are basically an opening in the side of an absorber tower. FIG. 1 shows the commonly offered inlet design without any protective awning. In this design, the liquid flowing off the absorber walls and any nearby spray headers falls on the inlet bottom plate causing solid growth known as "Elephant Ears". To overcome this problem, a designer places protective awnings on top of the inlet to divert the liquid curtain flow into the center of the absorber where gas humidification occurs with minimum contact between the hot gas and solid surfaces. This design as best seen in FIG. 2, has been proven functional at the traditional gas velocities and when the spray zone resistance is large enough to affect even distribution before the gas reaches the mist eliminator. As the gas velocity increases or the L/G decreases, however, the curtain resistance adds significantly to the overall system pressure drop and distortions to gas flow pattern becomes more critical. Lower liquid resistance in the upper portion of the absorber tower does not help gas redistribution.
U.S. Pat. No. 5,403,523 describes a device which deflects liquid slurry flow away from an inlet flue.
In view of the foregoing, it is seen that what is needed is a high velocity absorber having a combination of an advanced gas inlet design equipped with a flexible awning design capable of humidifying the entering gas, reducing the inlet pressure drop, controlling gas distribution across the absorber, and particularly for use when the spray zone resistance in the absorber is not adequate to correct gas flow distortions.